In elevator systems it has proved advantageous to use suspension-and-traction means that are composed of at least one electrically conductive steel rope and non-conductive sheath, or of ropes made of special plastics, in which an electric conductor is integrated. By this means, for the purpose of monitoring the individual suspension rope or ropes—also known as cords—a monitoring current can be applied. In the electric circuit so formed, or in several so-formed electric circuits, the current flow or current strength, the voltage, the electrical resistance, or the electric conductivity, is measured and provides information about the intactness and/or degree of wear of the suspension-and-traction means.
So, for example, the published patent application DE 39 34 654 A1 discloses a serial connection of all of the individual cords and an ammeter, or, instead of an ammeter, an electronic circuit, in which the base resistance of an emitter-connected transistor is measured.
U.S. Pat. No. 7,123,030 B2 discloses a calculation of the electrical resistance through a measurement of the momentary voltage by means of a so-called Kelvin bridge, and a comparison of the voltage value determined by this means with an input reference value.
International patent publication WO 2005/094250 A2 discloses a temperature-dependent measurement of the electrical resistance value, or of the electrical conductance, in which the varying ambient temperature, and hence also the assumed temperature of the suspension means, is taken into account, which, particularly in tall elevator hoistways, can greatly vary.
A further international patent publication, WO 2005/094248 A2, discloses special circuits of the individual cords, to avoid electric fields and to avoid orthogonally migrating ions between the individual cords.
A European patent publication, EP 1 275 608 A1, of an application by the same applicant as for the present application, discloses a monitoring of the sheath by application to the cords of a plus-pole of a source of direct current, so that in the case of a damaged sheath, a mass contact occurs.
However, disadvantageous in all of these known monitorings of the suspension-and-traction means is that the information about the signs of wear, or about the prevailing anomalous state of the suspension-and-traction means, is present only as an overall result. In particular, cross-connections (short circuits) between cords greatly falsify the overall result.